The investigators propose to measure the functional caliber of the alveolar septal microvascular syncytium in isolated rat lungs in zone 1. In zone 1, lung inflation pressure exceeds both pulmonary arterial and venous pressures and it has previously been assumed that this causes the alveolar septal microvessels to fully collapse. Numerous physiological studies are based on this assumption. Lungs can be perfused in zone 1, and the usual explanation is that this occurs by way of alveolar corner vessels, which bypass the septal microvessels. However, speculation is beginning to emerge that the septal microvessels may be at least partially perfusable in zone 1. If this is so it has significant implications for previous studies in which it was assumed that these vessels were collapsed. To address this, the investigators propose to measure the functional caliber of the alveolar septal microvascular syncytium in zone 1. The measurements will be made by perfusing isolated rat lungs with fluorescent latex particles of specific size (0.010 - 10.0 microns diameter). Lungs will be perfused under specific zone 1 conditions, with each lung being perfused with particles of a single diameter.The lungs will then be frozen rapidly, and histological samples will be examined using a computerized laser confocal fluorescence microscope. From digital images, latex particle density will be measured in individual alveolar septa. Data will be compared from lungs perfused with different particle diameters and used to calculate the functional septal microvascular syncytial caliber under the conditions in which the lungs were perfused. Lungs will be prepared in several combinations of perfusion and inflation pressures to determine how the septal syncytial caliber changes through zone 1 and during transition into zone 2, where the inflation pressure is less than the pulmonary artery pressure, but greater than the pulmonary venous pressure. Septal syncytial calibers will also be measured in liquid inflated rat lungs, where the surface tension of the septal air-water interface will be absent. The results will tell us if surface tension plays a role in maintaining septal perfusability in air inflated lungs in zone 1.